Coaxial cable with flat profile

ABSTRACT

A flattened coaxial cable structure has been found to exhibit advantages over conventional circular coaxial structures for some applications. This structure is a bifurcated conventional coaxial cable with the two semicircular segments joined by flat solid metal. A wide range of impedances can be realized without any significant penalty in added loss. Furthermore, tolerances to achieve certain design objectives, such as attenuation deviation, may be appreciably relaxed with the new structure.

United States Patent Miller et a1. [45] June 20, 1972 154] COAXIAL CABLEWITH FLAT 3,121,136 2/1964 Mildner 2174/28 PROFILE 1,912,794 6/1933Peterson .....333/96 X 3,106,600 10/1963 Crosby ..174/28 X [72]Inventors: Calvin Max Miller, Millersville; Robert 2 865 979 12 1958 74R Charles Sacks Emcon y both of M d Klassen ..l /117 [73] Assignee: BellTelephone Laboratories, Incorporated, FOREIGN PATENTS OR APPuCATlONSMurray Berkeley Heights, 956,334 1 1950 France 174/28 [22] Filed: Dec.16, 1970 Primary E.\'aminerLewis H. Myers PP N05 981787 AssistantExaminer-A. T. Grimley Attorney-R. J. Guenther and Edwin B. Cave [52]US. Cl. ..174/28, 174/16 B, 174/99 B,

174/117 F, 333/96 R ABSTRACT [51] Int. Cl. ..H0lb9/04 A flanened coaxialcable Structure has been found to exhibit held of Search g 1 advantagesover conventional circular coaxial structures for l 17 1 17 I Q b g ggggg 5 some applications. This structure is a bifurcated conventional lcoaxial cable with the two semicircular segments joined by flat solidmetal. A wide range of impedances can be realized [56] References Citedwithout any significant penalty in added loss. Furthermore,

UNITED STATES PATENTS tolerances to achieve certain design objectives,such as attenuation deviation, may be appreciably relaxed with the new2,356,166 8/1944 Lee et a1. ..174/l17 R structure. 3,136,965 6/1964Lunden ..174/27 X 2,297,202 9/1942 Dallenbach et a1. ..174/ 28 UX 6Claims, 16 Drawing Figures PATENTEDJum m2 3, 6T 1 ,662

SHEET 10F 4 PRIOR ART ,8 PRIOR ART F/G. 1c

CM. MILLER /NI/E/V7OR$ RC SACKS (MQ M ATTORNEY PATENTEDaunzo m2 3, 671.662

SHEET U 0F 4 "IIIIIIIIIIIIA rllll COAXIAL CABLE WITH FLAT PROFILE FIELDOF THE INVENTION This invention relates to coaxialcable structures in'general and particularly to such a structure comprising flattened topand bottom portions.

BACKGROUND OF THE INVENTION Numerous coaxial cable designs exist whichattempt to take advantage of the skin effect. The skineffectconsiderations prompted applicants to consider designs thatwouldincrease the effective surface area of the conductors at highfrequencies.

One object of the invention is to achieve savings in material costs in acoaxial cable to be used at high frequencies. I

A further object of the invention is to provide a coaxial cablestructure that at low frequencies exhibits a self-equalizing property.

SUMMARY OF THE INVENTION As will be shown in detail in the descriptionto follow, our coaxial cable structure is characterized by an outerconductor having rounded sides joined by flat, elongated top and bottomportions. This flat coaxial cable transmission line (FCC), is furthercharacterized by an inner and an outer conductor whose opposing surfacesare maintained at a uniform, constant mutual separation. This separationmay be denoted b-a where a is the radius of curvature of the outsidesurface of the inner conductor and b is the radius of curvature of theinside surface of the outer conductor. The opposing surfaces consist ofthe two semicircular end portions joined by flat portions of a width w.The advantageous operating range of the overall structure is specifiedby the relationships: b/a and w/b 1.5.

A feature of our invention, therefore, resides in the flattened,elongated top and bottom portions of a coaxial cable.

A further feature of the invention is in spacing arrangements for such acoaxial cable structure.

The invention, and its further objects, features, and advantages will bereadily discerned in full from a reading of the description to follow ofan illustrative embodiment taken in conjunction with the drawing.

DESCRIPTION OF THE DRAWING FIGS. 1A, 1B and 1C are schematic diagramsshowing the structural evolution of coaxial cable into the flat coaxialline 'ofFIG. 2, and FIG. 2A.

FIGS. 3-8 are graphical portrayals of various comparative relationshipsexhibited in circular coaxial versus flat coaxial lines.

FIGS. 9 and 10 are schematic cross-sectional diagrams of multiplejacketed flat coaxial structures; and

FIGS. ll, I2, and 13 are various schematic diagrams of spacers designedfor the present invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT FIG. 1A shows a crosssection of a standard coaxial line 1, and FIG. 18 a cross section of aparallel transmission line 2. FIG. 1C depicts the combining process oftwo parallel transmission lines 2a and 2b in a bifurcated coaxial linewhose sections are designated la and lb.

FIG. 2 depicts in cross section the resulting flat coaxial cable linedesignated generally 10. The line 10 consists of an outer conductor 11having an exterior surface 12 and an interior surface 13. The innerconductor is denoted 14, with exterior surface 14b.

As seen in FIG. 2, the top and bottom portions of both outer conductor11 and inner conductor 14 are flat within the region denoted w. Theradius of curvature of the two end portions of inner conductor 14 isdenoted a, and the radius of curvature of the interior surface of thetwo end portions of outer conductor 11 is denoted b. At either end,these curvature radii are struck from a common center, denoted 15 at theleft-hand end and 16 at the right-hand end. The resulting structure isshaped like a racetrack in which the opposing surfaces 13, 14bexperience a constantuniform separation along all points of thestructure.

The flat coaxial cable structure may also be thought of as a circularstructure in which the inner conductor has been hollowed out and thenboth inner and outer conductors flattened as described above. It will beappreciated that the flattening operation for cables having a diametergreater than a few tenths of an inch, does not increase the attenuationnearly as much asit decreases the areaof the inner conductor. Thematerial saved can be usedin part to increase the total area embraced bythe outer. conductor, which decreases the attenuation to less thanthefigure which obtained before the flattening. Hence, a lower attenuationand a net savings in cost of materials results. Of course, bothflattening and hollowing out achieve a further savings in materials.FIG. 2A shows a flattened coaxial cable structure with the innerconductor hollowed out, the hollowed out portion being denoted by thenumeral 14a.

The below equations are directed to high-frequency relationships thatshow attenuation increasing as the square root of frequency. At lowerfrequencies where f (2.6l4/a) and the conductors are electrically thin,flattened coaxial cable exhibits a self-equalizing property. That is,the attenuation is constant with frequency up to a transition frequency.This characteristic is potentially advantageous in digital systems wherea flat response would yield. good pulse transmission. The transitionfrequency can be increased by decreasing the thickness of the inner andouter conductors.

Multicoax cables consisting of plurality of flattened coaxial cables, asdescribed above, are depicted in FIGS. 9 and 10. In FIG. 9, a pluralityof flattened coaxial cable units or structures, each designated 20, aredisposed in a line with their inner conductor centers lying insubstantially the same plane. Advantageously, the edges of adjacentstructures are not in contact. An extruded jacket 21 of, for example,polypropylene, is placed over the flattened coaxial cable structures 20.The resulting ribbonlike structure will bend more readily than a similarstructure consisting of circular coaxial cables since for the same a and2 the thickness is less. Further, crosstalk is normally reduced becausethe inner conductor midpoints are spaced a greater distance than for thecorresponding conventional circular coaxial cable.

FIG. 10 depicts a multiplicity of flattened coaxial cable structures,denoted 20a, in a diamond configuration that approximates a circle incross section. The structures 200 are not in edge contact. An extrudedjacket 21a is placed around the assembly. In the configuration of FIG.10, the flattened portions of structures 20a advantageously are parallelto one another.

Spacing of the inner and outer conductors as well as support for theinner conductor are achieved pursuant to the following further inventiveembodiments.

In FIG. 11 the inner conductor 14 is spaced from the outer conductor 13by means of an insulating layer 30 which has been longitudinallyundulated or crimped in six places as shown. The layer is continuousadvantageously along the entire length of the structure, its crosssection being depicted as in FIG. 11. Specifically, two end crimps 30a,30b, contact the ends of the inner conductor 14. The opposing crimps30c, 30d, contact the top and bottom surfaces respectively of innerconductor 14 adjacent to the crimp 30a. Similarly, the crimps 30e, 30f,contact the top and bottom surfaces of inner conductor 14 adjacent tothe crimp 30b. The insulating material advantageously is a plastic suchas polypropylene.

A lattice-type spacer is depicted in FIG. 13. This spacer, denoted 40,comprises basically a pair of legs 41, 42 intersecting in latticefashion at their lengthwise midpoints. The respective ends 43, 44 and45, 46 are semicircular in shape, and are ofiset with respect to thelegs 41 so that the ends 43, 45 fall in a common plane and the ends 44,46 fall in another common plane which is parallel to that of ends 43,45. Lengthwise down the entire midsection of the legs 41, 42 are slits47, 48. These are slightly wider than the width of inner conductor 14.

As seen in FIG. 13, a number of spacers 40 are combined as end-to-endunits and mounted upon the inner conductor 14. The legs 41, 42 eachstraddle-mount the inner conductor 14, with their top and bottom sidesnormal to the top and bottom surfaces of inner conductor 14; and areoriented obliquely, rather than perpendicularly, to the central axis. Byvirtue of the oblique orientation of each leg 41, 42 with respect to theinner conductor central axis, it is seen that inwardly directed forceson the ends 43, 45 and ends 44, 46 bring the ends of slits 47, 48 intofirm contact with the edges of inner conductor 14. In this position, theends 43-46 are substantially perpendicular to the central axis and alsoto the flattened surface of inner conductor 14. The spacer provideshoneycomblike support of the outer conductor, as well as positivecentral spacing of the inner conductor with respect thereto.

The benefits of the inventive flat coaxial cable structure were furtherrevealed from the following analysis.

For coaxial and parallel transmission lines, the high p resistivity ofconductors in ohm meters f frequency in Hz t magnetic penneability ofconductors in henrys/meter e dielectric constant of insulation infarads/meter m 21rf Z, characteristic impedance R resistance inohms/meter G conductance in siemens/meter L inducatance in henrys/meterC capacitance in farads/rneter e, relative pennittivity a totalattenuation in neperslmeter a, loss due to conductors in nepers/meter aloss due to dielectric in nepers/meter A'I'I'ENUATION To exhibit theattenuation characteristics as a function of dimensions, Equation (3) isnormalized with respect to banda respectively. Thus The asymptoticconditions of 20:0 and w: a: result in These formulas represent thecases of coaxial line (w=0) and infinite parallel strip line w= areplotted respectively for the conditions e, 2.28 (for polyethylene) andp= 1.741 by 10" ohm meters (for copper). FIG. 3 particularlydemonstrates the interaction between the coaxial and the parallel linesin the intermediate dimension case. Notice that the standardoptimization conditions for coaxial are exhibited. The null regionbroadens until the hyperbolic form dominates. This is equivalent tovarying b/a holding b constant. FIG. 4, on the other hand shows a simplefamily of hyperbolics with no local minimal or maxima analogous tovarying b/a holding a constant.

IMPEDANCE The normalized equations for impedance from Equation 1 are:

FIGS. 5 and 6 are graphical representatives of Equations I0 and 11. Asin attenuation, these represent the case where b and a, respectively,are held constant. The conditions w=0 and w= lead to and coax in itsminimum loss region fixes its impedance to approximately 50 (I (forpolyethylene). In contrast, due to the broadening of the minimum lossregion for L0 w/b 1.5, a wide range of impedances can be obtained withFCC without a significant penalty in added loss.

EXACT REPLACEMENT CABLE teristic impedance given in Equation (I), on theother hand,

has no frequency characteristic and can be considered the infinitefi'equency asymptotic value.

DIMENSIONAL SENSITIVITIES It should be noted from FIGS. 3 and 5 thatoperating a circular where N 2 or 3. The implicationsof relation (16)can be seen in F IG. 7. The heavy curve represents the normal operatingrange for circular coaxial cables. The specific sensitivity coefficientsare:

in: S/(39 m normalized equations a, and a equations (4) and (5) may alsobe found. The relationships are:

ker K l k a K l (21) bla n b/ n b) where k, km, and k,, k To lend someadditional support to FIG. 7, it may be argued that the total percentuncertainty in attenuation is given by (-t.)= e) (th s Since the threeprocesses are independent, we plot the power sum'of the coefficientsassuming that the specification percent tolerances of all threedimensions will probably be equal.

From the foregoing, it is readily seen that the advantage gained usingthe FCC concept depends on its use in the operating range of b/a and w/b1.5. This is contrasted to the usual circular case where 3 b/a 5.

It is significant to note the results of dimensional tolerances oncharacteristic impedance. In many applications, computer cables forexample, impedance uniformity is much more important than attenuation.

Applying definition (14) to Equation (1) with Z, r) substituted for11(7) we obtain the following:

6, ere) In this case,

( In W (26) and N Lim 2102,:2 for N=2 or 3 Following the line ofreasoning culminating in Equation (23), FIG. 8 is a plot of the r.s.s.values of the sensitivity coefficients. For w 0, only the regionrepresenting coaxial in its normal operating range of b/a values isshown. Consequently, as before, it can be seen that it is reasonable tooperate FCC in regions (b/a l0 and w/b 1.5) where sensitivities arereduced both in attenuation and impedance.

In summary, the consequences of flattening coaxial cable in terms ofattenuation, impedance and manufacturability have been demonstrated. Thestructure possesses a versatility owing to the range of impedances thatcan be obtained while the cable is operating near an optimum point.Further, crosstalk is likely to be less, in general, between two flatcables placed side by side with their inner conductors coplanar. It isseen also that manufacturing costs stand a chance of reduction becauseflattened coaxial cable may be made by laminating strips rather than byextrusion. Also, high frequency integrated circuitry may benefit fromthin film flat cable for short runs.

It is to be understood that the embodiments described herein are merelyillustrative of the principles of the invention. Various modificationsmay be made thereto by persons skilled in the art without departing fromthe spirit and scope of the invention.

We claim:

1. A coaxial cable comprising:

an outer conductor having semicylindrical side portions each having aninterior surface radius of curvature b, and flat top and bottom surfaceseach of width w joining said side portions;

an inner conductor having semicylindrical side portions each having anexterior surface radius of curvature a, where b a, and flat top andbottom surfaces each also of width w joining said last-named sideportions;

where b/a 10 and w/b L5; and

spacing means for maintaining a uniform, constant material separationbetween said interior and exterior surfaces.

2. A coaxial cable pursuant to claim 1, wherein said inner conductorinterior is hollow.

3. A multicoaxial communications cable comprising plural coaxial cableunits constructed in accordance with claim 1, said units being disposedwith their said inner conductors lying in substantially the same plane,the edges of adjacent said units being in noncontacting relation, and aunitary outer jacket surrounding all said units.

4. A coaxial cable pursuant to claim 1, wherein said spacing meanscomprises a continuous insulative layer disposed between said outer andinner conductors, said layer comprising plural longitudinal undulationsextending from said outer conductor interior surface and contacting saidinner conductor midway of the latters semicylindrical side portions, and

also inwardly of both said side portions on both said top and bottomsurfaces of said inner conductor.

5. A coaxial cable pursuant to claim 1, wherein said spacing meanscomprises a plurality of units comprising first and second legsintersecting at their lengthwise midpoints, each leg having semicircularends and a central lengthwise slit uniform, constant mutual separationdenoted a-b, where a the radius of curvature of said inner conductor,and b the radius of curvature of the inside surface of said outerconductor; said surfaces comprising first and second semicircular endportions joined by flat portions of width w, and further characterizedin that b/a 10 and that w/b 1.5.

, UNITEDSTATES PATENT OFFICE CERTTLHCATE Q]? (IQIRRECTION Patent 3, 7 2Dated June 20", 19712" i Inventofls) Calvin M. Miller and Robert CASacks It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

' L lyi i Column 3, lin 33 q n 1, Change 5 to (Please note that thischange was requested in our I amendment received in the Patent Office onDecember 22, 11971.)

E I 11 2 11 2 Column 6, line 17, change 35- 00 t9 v 1 e v 1 Signed andsealed this 20th day of February 1973.

(SEAL) Attesjp:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCI-LALK 'Attesing Officer Commissionerof Patents FORM PO-1050 (10-69) 'uscoMwDc eoanmoa i 11.5 GOVEINMINIPIINYING CE 1!. O-JO6-JSI UNI EDSTATES PATENT OFFICE 5 CERTIFICATE (HFCQRRECTIQN y 1? Patent No. 3, 671, 662 Dated June 20', 1972'- Inventor sCalvin M. Miller and Robert c. Sacks 1 It is certified that errorappears in the above-identified patenti and that said Letters Patent arehereby corrected as'shown below: I L 1;; 5

Column 3, line 33, equation 1, change Z C .5 to

(Please note that this change was requested in our amendment received inthe Patent Office on December 22, 1 971.

v i-X Column 6, line 17, change to l 1 1 Signed and sealed this 20th dayof February 1975.

(SEAL) Attes't:

EDWARD M.FLETCHER,JR. v ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO-IOSO (0-69) v c'50375qa5g u.s. aovenunzuv PRINTINGarr-c: nu O-Jfi6-J.N

1. A coaxial cable comprising: an outer conductor having semicylindricalside portions each having an interior surface radius of curvature b, andflat top and bottom surfaces each of width w joining said side portions;an inner conductor having semicylindrical side portions each having anexterior surface radius of curvature a, where b < a, and flat top andbottom surfaces each also of width w joining said last-named sideportions; where b/a > 10 and w/b < 1.5; and spacing means formaintaining a uniform, constant material separation between saidinterior and exterior surfaces.
 2. A coaxial cable pursuant to claim 1,wherein said inner conductor interior is hollow.
 3. A multicoaxialcommunications cable comprising plural coaxial cable units constructedin accordance with claim 1, said units being disposed with their saidinner conductors lying in substantially the same plane, the edges ofadjacent said units being in noncontacting relation, and a unitary outerjacket surrounding all said units.
 4. A coaxial cable pursuant to claim1, wherein said spacing means comprises a continuous insulative layerdisposed between said outer and inner conductors, said layer comprisingplural longitudinal undulations extending from said outer conductorinterior surface and contacting said inner conductor midway of thelatter''s semicylindrical side portions, and also inwardly of both saidside portions on both said top and bottom surfaces of said innerconductor.
 5. A coaxial cable pursuant to claim 1, wherein said spacingmeans comprises a plurality of units comprising first and second legsintersecting at their lengthwise midpoints, each leg having semicircularends and a central lengthwise slit between the ends of each said leg formounting said inner conductor, said ends contacting, anD extendingnormally from the edges of said inner conductor, said units beingcombined in end-to-end relation as a honeycomb support for said outerconductor.
 6. A coaxial transmission line comprising: an inner and anouter conductor whose opposing surfaces are maintained at a uniform,constant mutual separation denoted a-b, where a the radius of curvatureof said inner conductor, and b the radius of curvature of the insidesurface of said outer conductor; said surfaces comprising first andsecond semicircular end portions joined by flat portions of width w, andfurther characterized in that b/a > 10 and that w/b < 1.5.